Various types of press machine are selected and used in accordance with the kinds of plastic working. The press machines may be roughly divided into two types, that is, a mechanical press that employs mechanical force as a drive source for driving the same and a hydraulic press that employs hydraulic pressure such as the pressure of oil or water.
Mechanical presses have high working speed and high productivity. Hydraulic presses, however, have the following superior features: the pressure application period can be increased; the pressure can be made to last longer; it is easy to adjust the level of pressure; and it can generate high pressure. As power transmission mechanisms for mechanical presses, mechanisms such as crank-, knuckle joint-, cam-, screw-, rack- and link-type mechanisms are known. Mechanical presses without flywheel have screw-, rack- or link-type power transmission mechanisms and many of them are small in size. The capacity to store energy is small, the stroke is determined in accordance with the resistance of a workpiece, and the bottom dead point is unstable.
Lead frames for ICs are produced by blanking a metallic sheet material. Press machines are used to generate pressure for this blanking. As press machines for this purpose, hydraulic presses are used in most cases. This is because it is necessary to detect a possible error of the work position by a sensor such as an inspection pin and suspend momentarily the press machine in order to prevent generation of defective products due to, for example, offset position of the work inside the blanking die. In other words, hydraulic presses are superior in controlling the performance such as stopping and starting, and therefore used for the above-described purpose.
There are several problems in pressing or in blanking a metallic sheet material.
First, there occurs breakage or damage to the workpiece when the tool leaves from the workpiece. This is because since the moment the tool impacts upon the workpiece it momentarily sticks upon the workpiece because of the momentum vacuum between the tool and the workpiece. Thus, when the tool leaves from the workpiece immediately after the completion of working, it also pulls workpiece up. Therefore, if the workpiece is a small and thin product such as a lead frame of an integrated circuit, the tool inevitably damages the workpiece.
If the retrieval speed is reduced to avoid this problem, then the productivity is also lessened.
Another problem is a noise of the press machine.
Hydraulic presses generate a considerably unpleasant noise including a pumping noise generated in a hydraulic pump, intermittent high-pitched noise generated when the hydraulic pipe for press working expands, solenoid operating noise, etc. In a hydraulic press, even when it is not working, as long as the switch for the hydraulic pump is ON, electric power is consumed more than while it is in an operative state, and it is in an operative state, and it is uneconomical. Mechanical presses also generate a high level of noise including noise generated when the flywheel is rotating, gear transmission noise, noise generated when the clutch is engaged and disengaged, and mechanical frictional noise generated due to backlash.
Further, in both hydraulic and mechanical presses, when a tool strikes against a workpiece for plastic working, an extremely high impact noise is generated, and also when the cut workpiece is separated from the tool, a noise (also referred to as "stripping noise" in this specification) is again generated. Thus, press machines have heretofore been synonymous with main sources of generation of a high level of noise in factories. For this reason, how to reduce the unpleasant noises in both hydraulic and mechanical presses has been very important subject for work-site people.
In general, noise in the factories or the like makes people not only feel uncomfortable but also mentally fatigued. As a result, the incidence of workmen's accidents increase and the working efficiency lowers. In addition, a hearing difficulty may be caused. If a person is engaged for a long time in an operation where he is exposed to a considerable noise, such as a press working operation, there is a fear of that he may suffer from occupational bradyacusia. It is necessary to take some measures from the viewpoint of workmen's health. Under these circumstances, standards have been established by academic societies and laws relevant to the subject on the basis of three factors, that is, the center frequency and the exposure time per day, so that no hearing disorder is caused.
For example, the Industrial Sanitary Society of Japan established a permissible noise level in 1975. According to this standard, the exposure time per day is limited to 2 hours at 85 phons. To meet this standard, it is a conventional practice to surround a press machine b, for example, a noise reducing panel formed in the shape of a box or lower the speed of operation. However, if a press machine is covered by a box-shaped structure, the external size increases and the view of the factory and the operability are extremely impaired. In addition, the factory space is restrained accordingly.